Whenever a communication system covering large distances is designed with pluralities of nodes with interconnecting signal traffic spans between source and destination, it has been important to know which span or which node is the source of a problem when communications fail. If the point of failure cannot be somewhat localized, the only recourse is to check the entire communication link from source to destination for the problem. This function of localizing the area of a failure for repair becomes even more important when the sophisticated communications networks of today allow a communication hookup to follow any of many different paths in accordance with circuit capacity available at the time of communication initiation. In other words, in making a call from Los Angeles to New York, a connection could be made such that the call might detour through either Seattle or Atlanta if a more direct connection through Kansas City, Mo. is fully loaded and cannot accommodate any more communication circuits.
Typically, in the prior art, the determination as to where a breakdown in communications has occurred is performed by a central intelligence which may periodically check the functioning of each span in an overall network using some type of communications to detect and localize the failure point, and report the problem along with an approximation of the point of failure so that repair crews can be dispatched to fix the communication link failure and/or communication can be rerouted around the failed portion.
When central controllers are utilized, there is already a knowledge of the sequence of nodes through which a circuit travels. Present day technology also provides for an alarm indication signal (AIS) as a replacement for the data communication signal. This AIS is generated by various intelligent nodes in the network, such as a cross-connect switch, detecting a directly observable signal impairment (DOSI). Such a DOSI might be complete lack of signal, loss of frame bits or some other easily recognized phenomenon. Each node receiving an AIS signal will pass it on to the next node. Since a central controller knows the sequence of nodes through which a circuit travels preceding the first node in a series which transmit AIS and which do not, the central controller can locate the fault as preceding the first node in series which outputs AIS.
Due to the complexity of using a central controller for large areas and the problems involved in network restoration on a substantially real-time basis, attempts are being made to provide distributed intelligence at each cross-connect in a communication network. The present invention utilizes a concept of using each intelligent node, such as a cross-connect, to monitor each circuit received for a directly observable signal impairment or a first format signal, such as the previously referenced alarm indication signal. If a node detects a directly observable signal impairment or a first format signal, it sends a second format signal downstream and times the length of reception of the DOSI or the first format signal. The second format signal could be something such as a fairly recently established IDLE signal as set forth by the American National Standard for Telecommunications. A given node declares failure of a channel in the span immediately upstream from that node when either the directly observable signal impairment or the first format signal is detected for longer than a predetermined period of time. It should be noted that an AIS signal can be generated in today's networks by devices other than intelligent nodes. One example of such a signal generating device is a fiber optic terminal. Fiber optic terminals are used to convert fiber optic signals to electrical signals for use in cross-connects, gateways or other devices in the network. These semi-intelligent boxes may generate the AIS signal due to loss of reception of the laser signal or due to some circuit failure within the fiber optic terminal itself. It will be realized that a cross-connect cannot instantaneously detect and change the lack of a signal or the first format signal to a different format signal. Thus, the lack of signal or the first format signal may pass through the entire circuit path and then in a ripple effect, the signal is changed to a different format signal by each intelligent node downstream from the device initially detecting a failure. Since each intelligent node in the network is known to provide such an alteration in the signal format, the existence of a DOSI or the AIS signal for longer than the time necessary for this change to ripple through the network provides a localization indication of the failure being in the span immediately upstream from the node detecting the problem for longer than a given predetermined period of time.
It is thus an object of the present invention to have a means of ascertaining at a local level or intelligent node level, the internode signal span having a failure mode.